The present invention relates generally to the field of wireless communication networks and in particular to a method of communicating the boundaries of higher layer data packets using the Radio Link Protocol (RLP).
The 3rd Generation (3G) wireless communication networks provide mobile users wireless access to packet data networks, such as the Internet. Many Internet applications and services, once available only to users at fixed terminals, are now being made available via wireless communication networks to mobile users. Services such as real-time streaming video and music, on-line interactive gaming, text messaging, email, web browsing and Voice over IP (VoIP), or Push-to-Talk (“walkie talkie” functionality) are just a few examples of services now being provided via wireless networks to mobile users.
These services are characterized by packet-switched data transfer, in which data is encapsulated into a logical unit called a packet, which contains a source and destination address and is routed from source to destination along nodes in one or more networks. Many data packets may be transmitted together on shared wireless traffic channels, with each mobile station retrieving only data packets addressed to it. This mode of data transfer is distinguished from the traditional circuit-switched paradigm of early-generation wireless voice communications, wherein a wireless traffic channel was dedicated to each individual call, or voice conversation. Packet-switched data transfer is generally more flexible and allows for more efficient utilization of network resources, than circuit-switched data transfer. However, data packets may also be transmitted on dedicated traffic channels.
According to some modem wireless communication network standards, a Packet Data Service Node (PDSN) within the network interfaces to external packet-switched data networks, such as the Internet, and effects Internet Protocol (IP) packet data communication between these external networks and the Radio Access Network (RAN) of the wireless system. Within the RAN, a Base Station Controller (BSC) eventually receives packet data forwarded by the PDSN, and directs it to individual mobile stations in radio contact with one or more Radio Base Stations. Packets are also communicated in the reverse direction, from a mobile station to an external network node.
On the wireless network side of the PDSN, under some current network standards a Point-to-Point Protocol (PPP) is established between the PDSN and the mobile station. The PPP protocol uses a High-level Data Link Control (HDLC) protocol link layer. The HDLC service encapsulates higher layer packets (HLP) into data link layer frames. The frames are separated by HDLC flags, or unique bit sequences that delimit the beginning and end of a frame. To prevent data within the frame, which may have the same bit sequence as a flag, from causing erroneous frame boundary determinations, flag-matching bit sequences within the HDLC frame payload are escaped and modified. That is, a second unique bit sequence, the escape sequence, is inserted, and the flag-matching bit pattern is modified, such as by XOR with a predetermined value. Any occurrence in the data of the escape sequence itself is also escaped and modified. This protocol makes the HDLC frame “transparent,” in that any sequence of data bits may be reliably transmitted.
At the receiver, each octet in the frame is inspected, and the data between two occurrences of the flag bit sequence are determined to comprise the HDLC frame. Additionally, the frame data is searched for the escape sequence. If found, the escape sequence is removed, and the following octet is XORed with the predetermined value, restoring the data to its original state. This need to inspect each and every received octet to detect either a frame-delimiting flag or an escape sequence is processor-intensive. The task may be delegated to hardware; however, this would impose a new requirement on equipment manufacturers, and require an upgrade of fielded equipment. An additional drawback of the HDLC framing protocol is that each occurrence of the escape sequence must be transmitted across the air interface, only to be removed by the receiver. This wastes scarce air interface resources.
In the Broadcast/Multicast Services (BCMCS) architecture, PPP, and hence, HDLC, is not utilized. In BCMCS, the framing protocol takes advantage of the traffic channel frame structure to transmit information regarding higher layer packet (HLP) framing. In particular, the framing protocol at the transmitter utilizes a predetermined number of bits at the beginning of the data in each Multiplexing Sublayer Protocol Data Unit (MuxPDU) to pass higher layer framing information. The bits indicate whether the data in the MuxPDU comprise a fragment of a HLP or a complete HLP. In the case of a fragment, the bits further indicate whether the fragment is from the beginning, middle or end of the HLP.
In the case where the MuxPDU is of a fixed size (e.g., BCMCS over a High-Rate Packet Data channel), a length field is also included at the beginning of the data in each MuxPDU. The length field indicates how much of the data in the fixed-size MuxPDU belongs to a particular HLP. Data from another HLP (with framing information bits included) or perhaps padding is added to fill the MuxPDU. In the case of a variable-size MuxPDU (e.g., BCMCS over CDMA2000-1X), the data in each MuxPDU contains only bits indicating framing information. No length information is included, as the MuxPDU header provides this information.
The receiver examines the beginning of the data in each MuxPDU received. It utilizes the framing information bits to determine whether the payload contains a complete HLP or a fragment of a HLP. In the case of fragments, the receiver utilizes the framing bits to re-assemble the HLP from data transmitted in multiple MuxPDUs. In the case of fixed-size MuxPDUs, the receiver also utilizes the length information bits to determine how much of the data in the MuxPDU belongs to a particular HLP. Since the framing and length (when present) information are positioned at the beginning of the data in each MuxPDU, the receiver can obtain this information efficiently, without having to parse all received data octets, as required in HDLC.
Although the BCMCS framing method is less processor-intensive than HDLC, it requires framing and length information to be sent in the data payload of every MuxPDU. For packet data services where RLP is utilized, the inclusion of the framing and length information results in at least one octet of RLP payload (or possibly more, depending on of the size of the length field) not being available to carry actual data, since the RLP payload consists of integer number of data octets. In many cases, the framing and length information in several of the RLP frames/MuxPDUs is redundant, as the same information is carried in several consecutive RLP data frames/MuxPDUs. For example, where the HLP spans several RLP data frames, all of the RLP data frames carrying data from the middle of the HLP (i.e., not the beginning or the end) carry the same framing information. This may occur, for example when a large HLP is being transmitted with a low data rate assigned to the air interface channel.
Framing methods that avoid the inefficiencies of HDLC framing will be necessary for the evolution of the CDMA2000 Packet Data Architecture. The BCMCS framing approach is an improvement over HDLC, but still consumes air interface resources to transmit framing and packet length information. Optimally, these resources should be reserved for user data to the maximum extent possible.